The present invention relates to a radial tire for motorcycle, more particularly to a cross-ply steel-cord belt structure suitable for a rear tire of a large sized motorcycle.
In the radial tires for four-wheeled vehicles, a cross-ply structure including two cross plies of steel cords is widely employed as a tread reinforcing belt.
In comparison with the tread portion of a pneumatic tire for four-wheeled vehicle, the tread portion of a pneumatic tire for motorcycle is convexly highly curved as a characteristic of a motorcycle tire. As a result, the belt cords are subjected to a larger deformation during running.
Therefore, if the steel belt cords usually employed in the passenger tires are used in a radial tire for motorcycle, due to the compression fatigue of the cord, the strength of the cord is rapidly decreased, and the durability of the tire is decreased. Therefore, an organic fiber cord having resistance to compression fatigue is usually employed in a cross-ply belt structure of a radial tire for motorcycle. In particular, from a point of view of handling stability, there is a tendency that aramid cords are used for example as disclosed in Japanese patent application publication No. JP-A-H04-365602.
Although the aramid cords have high elastic moduli, such moduli are not enough from a point of view of handling stability.
Japanese patent application publication No. JP-A-2008-100552 discloses a radial tire for motorcycle in which, in order to overcome the decrease in the durability due to compression fatigue, a steel cord made of very fine filaments of less than 0.10 mm dia. is used as the belt cord.
Such very fine filaments are however, costly and not readily available. Further, the handling stability is still not reached to satisfactory levels.